When an image fiber is used under a condition under which it is exposed to radiation, particularly gamma-ray, its attenuation is increased. Such the attenuation increase of the image fiber, however, can be minimized by forming it with optical fibers each comprising a cladding made of fluorine-doped silica glass and a core made of pure silica.
Even in such the image fiber, its attenuation is caused to seriously increase by gamma-ray in a relatively broad range near visible red light having a wavelength of 0.63 micrometer and, in some cases, transmission of image through the image fiber becomes impossible. This is a serious drawback of conventional image fibers, particularly when it is used for the transmission of visible light. Thus, in order to improve the performance of image fiber, it is essential to minimize the attenuation increase at or near the wavelength of 0.63 micrometer.
It is said that the attenuation near the wavelength of 0.63 micrometer is caused by defects in SiO.sub.2 glass, i.e. bond defects, which may be represented as follows: EQU Si--. . . --Si or Si--O . . .
wherein . . . represents a bond defect. Therefore, it is believed that the attenuation increase at or near the wavelength of 0.63 micrometer can be suppressed by removing the bond defects in the silica glass.
The image fiber has a further drawbacks that it has more bond defects than the usual silica glass. This may be because anisotropy is created in the longitudinal direction so that the bonds become easily broken during drawing of the image fiber preform (cf. for example, R. Bruckner, "Properties and Structure of Uitreous Silica, I", Journal of Non-Crystalline Solids, 5, 1970, 123-175).
Such defects are also found in an image fiber preform. However, the conventional image fiber or image fiber preform has not been treated so as to remove such bond defects.
It has now been found that such the bond defects in the image fiber can be removed by hydrogenating materials composing the image fiber not at the time of producing the core and cladding of the optical fiber, but after the production thereof to remove the bond defects, for example, by converting the bond defects such as Si--. . . and Si--O . . . into Si--H and Si--OH.